Manufacture of semiconductor structures requires highly precise and accurate metrology instruments. To this end, one of the important factors to be considered is calculation of an uncertainty of a metrology tool. Typically, this is based on the repeatability and reproducibility of a metrology tool, e.g. by performing measurements on a test sample of partially constructed wafer from a production line and estimating the static repeatability and long term reproducibility (precision). However, the repeatability and reproducibility of a measurement is meaningless if the measurement is wrong, while these techniques do not evaluate the accuracy of a metrology tool.
For example, U.S. Pat. No. 7,286,247 describes assessment and optimization for metrology instrument including determining total measurement uncertainty. According to this technique, a total measurement uncertainty (TMU) is determined based on precision and accuracy. The TMU is calculated based on a linear regression analysis and removing a reference measuring system uncertainty from a net residual error. The TMU provides an objective and more accurate representation of whether a measurement system under test has an ability to sense true product variation.